TECTONICS AS THE CONTROL OF LONG-TERM CLIMATE CHANGE

Although Milankovitch orbital variations control the pulse of the climate system, the longer term trends are determined by tectonics. The relations between tectonics and climate are both direct and indirect. The direct connections are between plate motions and uplift which affect atmospheric and oceanic circulation. The indirect links are via subduction and volcanism which introduce greenhouse gasses into the atmosphere, and through erosion which alters the balance of atmospheric greenhouse gasses by chemical weathering and burial or organic carbon. Rifting of continental blocks involves an initial broad upwarp, followed by phase of brittle fracture resulting in subsidence of a central valley and uplift of marginal shoulders. The resulting evolution of the regional climate has been repeated many times in the Phanerozoic. Initially there is a vapor-trapping arch, followed by development of a rift valley with fresh-water lakes, culminating in an arid rift bordered by mountains intercepting incoming precipitation. As the rift subsides below sea level it often becomes a site of evaporite deposition. During the Phanerozoic this process has removed salt from the ocean faster than it has been supplied lowering the global ocean salinity and promoting a different behavior of the oceanic thermohaline circulation. Convergence tectonics affects climate on a larger scale. A mountain range is a barrier to atmospheric circulation, especially if perpendicular to the circulation. The geochemical effects of tectonic uplift and unroofing of igneous and metamorphic silicate rocks which then become exposed to weathering removing CO2 from the atmosphere-ocean system. Uplift also results in erosion and large scale burial of organic carbon in rapidly deposited sediments. Tectonics can also affect the connections between ocean basins, resulting in large scale changes of the ocean circulation. Interocean gateways can open and close in relatively short timespans and may induce major steps in the climate evolution of the Earth.